In oil well perforating operations, it has become common practice to use well perforating guns whose explosive components or shaped charges are directly exposed to the well bore fluids. Typically, these explosive components may be carried along a flexible or semi-flexible strip of metallic carrying member and may be locked to the carrying member and affixed through holes bored in that strip, thus directly exposing the shaped charges to the well bore fluid. Rather than flexible strips, another type of disposable or expendable perforating gun may incorporate a plurality of links, similar to the links of a chain, each having a hole bored therein for retaining a shaped charge member held by a retaining ring and pin or screw arrangement.
Guns of these types typically may be referred to as expendable perforating guns in the sense that, when the explosive charges are fired, the carrier of flexible material or linked chain-like material is destroyed or separated into numerous small pieces by the explosion of the shaped charges and may fall to the bottom of the well borehole as small debris. Such expendable carrier perforating guns have the advantage that the maximum sized shaped charge for a given diameter of carrier can be installed as opposed to conventional hollow carrier shaped charge perforating guns. The prior art hollow carrier shaped charge guns of course carry a plurality of longitudinally disposed shaped charges distributed along the length of a hollow mandrel or carrying tube which protects these charges from the borehole fluid environment.
Typically these expendable perforating guns are run on an electrical wireline having either a single or multiple electrical conductors and may be strung together or stacked one above the other to provide several gun sections to increase perforating capability. Such expendable carrier guns have in the past been fired by electrical signals sent down the wireline from the surface which detonate an initiator or explosive cap device which in turn detonates a secondary explosive detonating cord. The detonating cord then initiates the attached shaped charges. After the gun is fired, the wireline is retrieved to the surface of the well while the expendable carrier has been deposited at the bottom of the well bore as small sized debris at which time the well is ready to be produced through the perforations formed by the expendable shaped charge carrier.
The three explosive components that comprise an expendable perforating gun assembly are the detonator or initiator, the detonating cord and the shaped charges themselves. In order for these explosive components to function properly, they must be environmentally protected from well bore fluids, and the downhole pressures exerted by these fluids. Temperature effects on the explosive components may be accounted for by proper selection of thermally stable explosives; in wells up to 18,000 or 20,000 feet in depth, temperatures may reach 400.degree. F. and pressures may reach 18,000 to 20,000 psi. Therefore, the environmental protection provided to the explosive components must protect the components individually, and also the interfaces between the components that form the explosive train.